Having multiple phone numbers is helpful for a lot of reasons. Personally, I use my Google Voice number when filling out any potentially spammy forms or when selling things on Craigslist. I like being able to easily screen and block callers, as well as get voicemail transcriptions.
I also used to give it out to people after determining whether they were an iPhone or non-iPhone user — if they were an iPhone user, I gave them my real phone number so we could use iMessage…if they were not, I gave them my Google Voice number. The reason was not to be phone elitist, but that I used to be on that AT&T unlimited data plan and didn’t have free texts. Now that I’ve switched to T-Mobile (to save a LOT of money and have free international data), I no longer need to make that distinction.